1. Field of the Invention
The present invention concerns a magnetic resonance system, of the type having a basic magnet that operates a static basic magnetic field, in an examination volume, a whole-body antenna that emits a radio-frequency field in the examination volume, the radio-frequency field exhibiting an excitation frequency so that nuclei in an examination subject in the examination volume are excited to cause emission of magnetic resonance signals, and a radio-frequency shield between the whole-body antenna and the basic magnet, and wherein the whole-body antenna is arranged between the radio-frequency shield and the examination volume.
2. Description of the Prior Art
Magnetic resonance systems of the above type are generally known. In addition to the aforementioned components they normally have a gradient coil arrangement by means of which a gradient field is generated in the examination volume. The gradient coil arrangement is arranged between the basic magnet and the radio-frequency shield.
The radio-frequency shield has the goal of forming shielding currents in the radio-frequency shield in a predetermined manner in the transmission mode (thus when the whole-body antenna emits the radio-frequency field) such that the homogeneity of the radio-frequency field is maintained. The coupling of the radio-frequency field in the gradient coil arrangement should furthermore be prevented if a gradient coil arrangement is present. In the acquisition (reception) mode (thus when magnetic resonance signals are acquired from the examination volume by means of the whole-body antenna) the radio-frequency shield should prevent possible disruptions in the operation of the whole-body antenna emanating from the gradient coil arrangement.
In the ideal case, the radio-frequency shield is theoretically fashioned as an integrated, highly electrically conductive structure. In practice this embodiment is not possible since in this case high eddy currents would be induced by the gradient coil arrangement in the radio-frequency shield. The radio-frequency shield is therefore typically fashioned as a multi-level layer structure, with the layers being alternately electrically conductive and electrically insulating. The electrically conductive layers are slit. This embodiment of the radio-frequency shield (which is frequently used in practice) represents a compromise between various requirements.
It is advantageous to optimize the radio-frequency properties of the radio-frequency shield. This is primarily because in the course of the general development of magnetic resonance systems, the separation (spacing) of the radio-frequency shield from the whole-body antenna continuously decreases and the radio-frequency shield therefore always influences the properties of the whole-body antenna more strongly.